Catheter delivery systems for trans-luminal delivery of self-expanding stents have a rich history in the patent literature. Early proposals were for a simple sheath radially surrounding the radially-compressed stent at the distal end of the catheter system, the sheath being pulled back proximally, to release the stent from its bed, progressively, starting at its distal end of the bed, within the stenting site or stenosis of the bodily lumen in which the catheter delivery system had been advanced. Readers will appreciate that, because the stent is self-expanding, it is pressing on the luminal surface of the surrounding sheath, up to the moment of its release from the sheath. Thus, friction forces between the stent and the surrounding sheath must be taken into account when devising a delivery system that will allow the sheath to slide proximally over the full length of the outwardly-pushing, self-expanding stent.
The problems of friction will increase with the length of the stent, and the pressure on delivery system designers is to deliver ever-longer stents. Furthermore, there is steady pressure on stent delivery system designers to come up with systems that have ever-smaller passing diameters at the distal end of the catheter. The conventional unit of dimensions for diameters of systems to advance along a bodily lumen is the “French” which is one third of a millimeter. Thus, one millimeter is “3 French”. To be able to reduce the passing diameter of a delivery system, for example from 7 French to 6 French, is a notable achievement.
One way to respond to the challenge of friction forces between a proximally withdrawing sheath and a self-expanding stent confined within it is to adopt a “rolling membrane” sheath system, in which the sheath is at least double the length of the stent that it surrounds, being doubled back on itself at a point distally beyond the distal end of the stent. Then, proximal withdrawal of the radially outer doubled back portion of the sheath length will cause the “rolling edge” between the outer and inner sheath portions to retreat proximally, rolling proximally down the length of the stent, to release the stent progressively, as with a single layer surrounding sheath.
While the rolling membrane approach might solve the problem of friction forces between the proximally retreating sheath and the stent radially inside it, it replaces that problem with another friction issue, namely the need for sliding of the cylinder of the outer sleeve of the sheath over the abluminal surface of the remaining inner sleeve of the sheath that continues to radially constrain the stent within it. It has been proposed to provide a lubricant between the inner and outer radial portions of a rolling membrane release system, but designers would prefer, if possible, to keep to a minimum the use of any extraneous powder or fluid, including lubricants, at the distal end of a catheter. Further, there is the practical difficulty of incorporating into a manufacturing system a step of distributing lubricant as required, consistently and reliably and economically.
Consistency is important, because of the importance of certainty that, when the medical practitioner takes the decision to deploy the self-expanding implant at the distal end of such a catheter delivery system, the components of the delivery system will form as anticipated, every time, to release the implant smoothly and reliably, in the same manner every time. Any sort of unpredictable friction force is anathema to this objective. Hence, designers of these delivery systems will make every effort to minimize the unpredictable effects of friction on the release performance of their system. This is a tough challenge, particularly with the ever-present pressure to accommodate longer stent lengths and smaller passing diameters.